1. Field of the Invention
The present invention relates to a liquid crystal display (hereinafter, referred to as LCD) device including a liquid crystal layer interposed between two substrates and to a method for producing the same.
2. Description of the Related Art
An LCD device is produced by sealing a space between two substrates opposed to each other except for a portion acting as an injection opening and then injecting a liquid crystal into the space from the injection opening. Such an injection is conventionally performed by a liquid crystal injection apparatus which includes a liquid crystal bath in a chamber in which an inner pressure is adjustable. For example, the injection is performed in the following manner. The inner pressure of the chamber is reduced to approximately 10.sup.-3 torr by a vacuum pump, and then an injection opening is immersed in a liquid crystal in the liquid crystal bath with the injection opening being directed downward. By increasing the inner pressure to an atmospheric level, the liquid crystal is injected between the two substrates sealed with a sealing member. Finally, the injection opening is coated with an ultraviolet curing resin by a dispenser or the like and is sealed by radiating ultraviolet to the ultraviolet curing resin.
Such an injection is simultaneously performed for a plurality of LCD devices, and the injection openings thereof are sealed after the liquid crystal is injected into all the LCD devices. Namely, the injection is continued until all the LCD devices are filled with the liquid crystal. However, the optimum injection period is different depending on each LCD device. Accordingly, an LCD device to which the liquid crystal has been injected for a longer period of time than an optimum injection period thereof obtains a thicker gap between the two substrates than an LCD device to which the liquid crystal has been injected only for an optimum period thereof. (Hereinafter, the above-mentioned gap will be referred to as the cell thickness.) As a result, there is a variation in the cell thickness among the LCD devices, which prevents images from being displayed as designed.
Such a variation in the cell thickness causes a variation in colors displayed by the LCD devices as well as in the response time and the threshold level of the voltage for switching the liquid crystal between a display state a non-display state. Such inconveniences lower the display quality of the LCD devices. The cell thicknesses of all the LCD devices are uniformized by interposing spacers each formed of a plastic bead, a glass fiber or the like between the two substrates. However, the alignment of liquid crystal molecules is impaired in the vicinity of the spacers, thereby lowering the display quality.
FIG. 1 is a plan view of a conventional LCD device 61, and FIG. 2 is a cross sectional view thereof along lines G--G of FIG. 1. The conventional LCD device 61 includes a first substrate 62 and a second substrate 63 both formed of a material which transmits light such as glass, and a liquid crystal layer 66 interposed between the first and the second substrates 62 and 63 and sealed with a sealing member 64. Spacers 65 formed of a resin or glass in a shape of a ball or a cylinder are provided in the liquid crystal layer 66 and the sealing member 64. The spacers 65 are adhered to the first and the second substrates 62 and 63, thereby determining the cell thickness, namely, the thickness of the liquid crystal layer 66.
Inner surfaces of the first and the second substrates 62 and 63 on the side of the liquid crystal layer 66 each have an alignment film and pixel electrodes thereon. The pixel electrodes are provided for applying a voltage to the liquid crystal for pixels by a simple matrix operation or an active matrix operation. The alignment films are provided for determining the alignment of liquid crystal molecules of the liquid crystal layer 66. When necessary, a color filter layer and a light shielding layer are further provided. In the case when an active matrix operation is performed, a switching device for controlling the voltage application to the pixel electrode is constituted by a thin film transistor (hereinafter, referred to as the TFT) formed of an amorphous silicon or a polysilicon and is provided on one of the inner surfaces.
A method for producing such a conventional LCD device 61 will be described.
First, the first and the second substrates 62 and 63 having the pixel electrodes and the alignment films thereon are prepared. A perimeter of the inner surface of the first substrate 62 is coated with the sealing member 64 such as a thermosetting resin or a photocuring resin by use of screen printing or the like except for an injection opening 67. Then, the spacers 65 are scattered by use of wet scattering or dry scattering in the ratio of 100 to 150 spacers 65 per 1 mm.sup.2.
The first and the second substrates 62 and 63 are assembled after positioning, and then the sealing member 64 is cured by heating or ultraviolet radiation. In this manner, a liquid crystal panel is produced.
Then, an air pressure in the liquid crystal panel is reduced to a vacuum level, and the liquid crystal is injected into the liquid crystal panel through the injection opening 67. The injection opening 67 is sealed with another sealing member 68, thereby obtaining the LCD device 61. Thereafter, the first and the second substrates 62 and 63 are shaped as desired and cleaved, and a polarizing plate and a light reflection plate are added as necessary.
Such a conventional LCD device 61 has the following problems since the spacers 65 are provided in the liquid crystal layer 66. (1) When being scattered between the first and the second substrates 62 and 63, the spacers 65 tend to flocculate in a unit of several to several ten pieces. This phenomenon prevents the thickness of each liquid crystal layer from being uniform or lowers the transmittance of a portion of the liquid crystal layer 66 including the flocculated spacers 65, resulting in defective display. (2) The alignment of the liquid crystal molecules is impaired in the vicinity of the spacers 65, thereby lowering the optical modulation efficiency thereof, or light is leaked due to the transmittance of the spacers 65, thereby causing defective display or inferior contrast. (3) Since the cell thickness is adjusted by the injection period of the liquid crystal, simultaneous injection of the liquid crystal into a plurality of liquid crystal panels brings variation in the cell thickness among the liquid crystal panels due to errors in assembling the first and the second substrates 62 and 63 or errors in processing precision. Such a variation in the cell thickness causes inferior contrast of the LCD device. These problems are especially remarkable when a display image is enlarged in a projection apparatus using such an LCD device.
FIG. 3 shows another conventional method for assembling substrates for use in an LCD device.
The inner surface of the second substrate 63 is coated with the sealing member 64 (formed of a thermosetting resin) having the spacers 65 mixed therein by use of screen printing along a sealing pattern, and the spacers 65 for controlling the cell thickness are scattered on the entire inner surface of the first substrate 62. The number of the spacers 65 to be scattered is approximately 100 to 150 pieces per 1 mm.sup.2. Then, the two substrates 62 and 63 are assembled into a liquid crystal panel. The liquid crystal panel is set in a pressing machine including an upper surface plate 67 and a lower surface plate 69 and is applied with a pressing load so that the second substrate 63 is entirely pressurized with a uniform force. In this state, the sealing member 64 is cured by heating. By such a method, the cell thickness is controlled by the spacers 65 mixed in the sealing member 64 and the spacers 65 scattered on the first substrate 62.
In such a conventional method for producing the LCD device, a process of scattering the spacers 65 is indispensable to obtain a uniform cell thickness. In order to press the sealing member 64 into a thickness equal to the spacers 65 mixed therein, the pressing load is required to be large. In order to support the load and further maintain a uniform cell thickness, the spacers 65 are required in a large number. For these reasons, the scattering of the spacers 65 involves the following problems. (1) The ratio of a total area of pixels with respect to the display area is lowered due to a large number of spacers 65 existent in the display area, resulting in inferior display quality and contrast. (2) A luminance point is generated since the spacers 65 flocculate in a unit of several to several tens of pieces. (3) A pixel defect is generated by the breakage of the switching devices such as TFTs caused when the first and the second substrates 62 and 63 are pressed by the pressing machine.
In a case when the spacers 65 are not used, the display area has no medium for controlling the cell thickness. In such a state, the display area tends to be concaved or convex, thus preventing the cell thickness of each liquid crystal panel from being uniform. Accordingly, the display quality is lowered. In a case when the LCD device is as large as having a side of 25 mm in length, it is significantly difficult to obtain a uniform cell thickness.
Conventionally, the LCD device having no spacer has a size of approximately 1 inch. If, after the liquid crystal is injected into a vacuum liquid crystal panel in a conventional manner, the inner pressure is increased to an atmospheric level at one time, the two substrates are both bent due to the difference between the inner pressure and the outer pressure of the liquid crystal panel. Such bending is not a serious problem in the case of an LCD device having a size of approximately 1 inch. In the case when the LCD device has a size of more than 1 inch, if a height of the bending portion exceeds the cell thickness, the two substrates contact each other. This phenomenon causes the following problems. (1) The switching devices such as TFTs are broken, or the alignment films formed on the inner surfaces of the substrates are damaged to cause inferior alignment. (2) Since it is difficult for the liquid crystal to be injected into the liquid crystal panel, it takes an excessively long period of time until the liquid crystal is fully injected. If the substrates are not finally separated, defective injection occurs.